Update references of master branch to main branch

CONTRIBUTING.md

@@ -28,7 +28,7 @@ Check first if the bug might already exist: https://github.com/mozilla/contain-f
 Patches should be submitted as pull requests. When submitting patches as PRs:
 
 - You agree to license your code under the project's open source license (MPL 2.0).
-- Base your branch off the current master (see below for an example workflow).
+- Base your branch off the current `main` (see below for an example workflow).
 - Add both your code and new tests if relevant.
 - Run `npm test` to make sure all tests still pass.
 - Please do not include merge commits in pull requests; include only commits with the new relevant code.

docs/container-management.md

@@ -16,7 +16,7 @@ To **remove** a site from the Facebook Container, you can do the inverse, by nav
 
 Additionally, you can manage all the custom sites that have been added by clicking on "Sites Allowed in Facebook Container".  
 
-_*Note that [Facebook-owned domains](https://github.com/mozilla/contain-facebook/blob/master/src/background.js#L7-L23)_ cannot be removed. 
+_*Note that [Facebook-owned domains](https://github.com/mozilla/contain-facebook/blob/main/src/background.js#L7-L23)_ cannot be removed. 
 
 Once on that panel page, click the `X` next to the domain you would like to remove. 
 

src/background.js

@@ -567,7 +567,7 @@ async function containFacebook (request) {
 }
 
 // Lots of this is borrowed from old blok code:
-// https://github.com/mozilla/blok/blob/master/src/js/background.js
+// https://github.com/mozilla/blok/blob/main/src/js/background.js
 async function blockFacebookSubResources (requestDetails) {
   if (requestDetails.type === "main_frame") {
     tabStates[requestDetails.tabId] = { trackersDetected: false };